Superheterodyne receiver



April 21`,A 1936. D. E. HARNETT SUPERHETERODYNE RECEIVER Filed may 31,1934 com om. om. QN.- o. cm.-

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INVENTQR n @4N/EL .5. H1/FME 7 f ATTORNEYS www Patented Apr. 21, 1936UNITED STATES SUPERHETERODYNE RECEIVER Daniel E. Harnett, Tuckahoe, N.Y., assignor to Hazeltine Corporation Application May 31, 1934, SerialNo. 728,437

11 Claims.

This` invention relates vto the reception and selection of modulatedcarrier signals and has for its principal object-to improve the delityof reception. Y

A radio broadcast signal is ordinarily transmitted on a carrier wavehaving two sidebands of modulation frequencies. Under presentbroadcasting conditions, the different carrier frequencies are allocatedat various positions throughout ir the broadcast range, usually 10kilocycles apart,

and in many instances the sideband vfrequencies of one signal channeleither overlap those of an adjacent Vsignal channel or else closelyencroach upon them. VIn either case, it is difficult, when -tuning aradio broadcast receiver to a desired signal in one such channel, toeliminate interference due to signals in the adjacent channels,particularly when such interfering signals are received with a strengthcomparable to that of the 20. desired signal.

Operation without interference in such cases ordinarily requires thatthe selecting system shall pass a considerably narrowed band offrequencies so that passage of the interfering signals-is substantiallyprevented. But narrowing the selectedV band in this manner ordinarilyimpairs the fidelity of reception of the signals, since the sidebandfrequencies corresponding to the higher audio frequencies of modulationare suppressed. According to this invention, the fidelity of receptionis improved by employing a frequency selecting system by which theselected band may be extended in one direction or the other. This hasthe effect of admitting a wider range of one of the sidebands. Thedirection in which the selected band should ordinarily be extended isthat opposite from which a strong interfering signal on a nearby channelmay happen to be present.

A feature of the invention is the use of two or more selecting systemstuned to select a relatively narrow band of frequencies, one of thesesystems being fixed and another being variable. The variable system isprovided with means for shifting its resonant frequency somewhat ineither direction.

Another feature is the use of double-tuned circuits wherein adjustablereactive elements are employed in each circuit to expand the selectedband by any desired amount, in either direction, up to the limit ofexpansion.

The invention is very well adapted for use in the intermediate-frequencyselecting circuits of superheterodyne receivers.

Referring to the drawing:

Fig. 1 illustrates a superheterodyne receiver embodying the invention;and

Fig. 2 shows graphically how the selected band is extended in onedirection, in accordance with this invention.

The receiver of Fig. 1 is of a conventional superheterodyne typecomprising an antenna IIJ and ground II; a radio-frequency amplifier I2;a modulator tube I3; an intermediate-frequency amplifier tube I4; adetector and audio-frequency amplier I5; and a loudspeaker I 6. Theradiofrequency amplifier I2 and the detector and audio-frequencyamplifier I5 are shown in generalizedv box form since their details arewell understood in the art and constitute no part of this invention. Themodulator tube I 3 is coupled to the radio-frequency amplifier I2 by aradiofrequency coupling system I'I tunable by a variable condenser I8.There is provided a local oscillator indicated generally by the box I9having an loutput circuit 20 tunable by a variable condenser 2|. Theoutput of the oscillator is coupled to a coil 22 which is located in thegridcathode circuit of the modulator tube I3.

In accordance with well-known modulator operation, modulation of theyradio-frequency signals by the local oscillations occurs at the tube I3.

There consequently appears at the output of the modulator theintermediate-carrier frequency which is the difference between thesignal-carrier frequency and the local oscillator frequency. Theintermediate-carrier frequency has associated therewith the usual twosidebands of modulation which extend six or more kilocycles on eitherside of the intermediate-carrier frequency. To maintain theintermediate-carrier frequency substantially constant, the localoscillator frequency is varied by the condenser 2| by the same amountand in the same direction, that the resonant frequency of theradio-frequency selecting circuit is varied by the condenser I 8. Thetwo condensers I8 and 2| are ordinarily operated by a single mechanicalcontrol device indicated generally by the dotted lines U.

'I'heV intermediate-carrier frequency amplifier tube I4 is coupled tothe output of the modulator tube I3 by an intermediate-frequencyselecting system 23 of the double-tuned type. This selecting systemcomprises a primary coil 24 electromagnetically coupled to a secondarycoil 25. The primary coil 24 has shunted across it a fixed condenser 26.and a variable condenser'Z'I. The secondary coil 25 has shunted acrossit a fixed condenser 28 and a variable condenser 29. The two variablecondensers 21 and 29 are operated simultaneously by a single mechanicalcontrol device indicated generally by U. The mechanical control deviceis arranged so that operation thereof varies the capacities of the twocondensers 21 and 29 in the same direction.

Another intermediate-frequency selecting system 30 serves to couple thetube I4 to the detector and audio amplifier I5. This latter selectingsystem likewise comprises a primary coil 3l coupled to a secondary coil32, these coils being tuned to the intermediate-carrier frequency bycondensers 33 and 34, respectively, so that a relatively narrow bandcentered at the intermediate carrier frequency is selected. Couplingsystem 30, however, is not provided with variable condensers likecondensers 21 and 29 of selecting system 23.

Referring to system 23, the primary and secondary circuits are eachadjusted to tune to the intermediate-carrier frequency (frequently inthe neighborhood of klocycles) whenv the mechanical control device U' isin an intermediate posiiton between the minimum and maximum limits ofits range of operation. Preferably, the mid-range position of the deviceis chosen as that in which the system is so tuned. This position of U',then, is the neutral, or normal position in which the system is tuned toresonance at the intermediate-carrier frequency. In this neutralposition the variable condensers 21 and 29 ,are adjusted to havecapacity values intermediate their maximum and minimum values. Thesecapacity values, in the neutral position, should preferably be such thatthe resonant frequency can be shifted from the intermediate carrierfrequency in both directions by an equal amount.

The condensers 21 and 29 are so correlated by the device U that movementof U produces equal changes of the resonant frequencies of the primaryand secondary circiuts of the coupling system 23. Hence, these primaryland secondary circuits are always tuned to the same frequencythroughout the range of operation of device U. In the convenient casewhere coils 24 and 25 are equal and condensers 26 and 28 are equal,condensers 21 and 29 may also be equal and made to have their capacitiesvaried equally by movement of device U.

In the neutral or normal condition, the bandselecting characteristic ofthe selecting system 23 is the same as that of the succeeding selectingsystem 30, and is represented graphically by curve A in Fig. 2, which isa graph of relative output voltage on a logarithmic scale plottedagainst frequency on a linear scale. The intermediate carrier frequencyis taken to be 175 klocycles. Since the systems 23 and 30 both have thecharacteristic A, the overall characteristic of the two systems is asrepresented by curve B, which is the square of curve A.

An inspection of curve B indicates that when the condensers 21 and 29are in the neutral position the system is highly selective. If theselected band width be taken as that at which the relative outputvoltage is half the peak voltage, the band width passed is less than 4klocycles wide, that is, less than 2 klocycles on each side of thecarrier.

In accordance with the usual desired tuning operation the receiver istuned with the main tuning control device U with the control device Uset in the most selective, or neutral, position. After the desiredsignal is accurately tuned in, the device U' is adjusted to expand theband width the desired amount on the side away from the strongestinterference.

When the control device U' is moved to decrease the capacity ofcondensers 21 and 29, the resonant frequency of the selecting system 23is shifted upward with respect to the intermediatecarrier frequency. Foran upward shift of 5 kilocycles in the resonant frequency of system 23the characteristic shifts from that of curve A to that of curve C. Theoverall relative voltage output characteristic now takes the form ofcurve D which is the product of curves A and C. An inspection of curve Dshows that the overall resonance peak has shifted about 21/2 klocycles,and that the band width has increased to about 6 kilocycles.

It is significant to note that the greatest shift has occurred at thehigh-frequency side of curve D, the low-frequency side having shiftedupward very little. Consequently, most of the increase in band width hastaken place in the region of the upper sideband. Hence, a much widerrange of the upper frequency sideband is selected under the condition ofcurve D than under the condition of curve A, the selected portion ofthis sideband having increased from less Athan 2 klocycles to more than5 klocycles. This means that the audio-frequency range has beencorrespondingly increased.

The width of one sideband, and hencethe audiofrequency range, may befurther extended by increasing the detuning of system 23 from theintermediate carrier frequency. Curve E shows the characteristic whenthe system 23 is detuned by 12 klocycles from the intermediate carrierfrequency. 'I'he greater output at the peak near 175 klocycles, relativeto the peak near klocycles is due to the added selectivity of the radiofrequency selecting system which is effective near frequenciescorresponding to 175 klocycles. For the simplicity of explanation theeffect of the radio frequency amplifier is not shown in curves A, B, C,and D.

It may often be desirable to increase the number of intermediatefrequency tuned circuits. This may be done by utilizing two intermediatefrequency amplifier tubes in tandem with three double tuned couplingsystems like systems 23 and 30, that is six intermediate frequency tunedcircuits. In the latter case, one of the systems may be designed likesystem 23 of Fig. 1 and a second of the systems may be designed todetune only one of the two tuned circuits thereof. In such a case, itwill be convenient to use a threegang midget condenser having two likesections for the doubly adjustable system and one smaller capacitysection for the coupling system having only one adjustable circuit. Suchan arrangement would have the advantage of tending to flatten out thetwo peaks shown in curve E of Fig. 2, since the said smaller capacitysection will not shift the resonance of its tuned circuit as far as theresonance of the doubly adjustable circuits are shifted. Hence thecircuit of the said smaller capacity section will be resonant betweenthese two peaks.

As an alternative arrangement, it would be possible to use likecondenser sections but different inductance values. For example, thedoubly adjustable system such as 23 could be constructed to have equalinductances, whereas the second coupling system having only one of thetuned circuits adjustable could have the inductance of the adjustablecircuit somewhat smaller than the other inductance of the same system.

` A similar eifect may be obtained byv using siirv tunedA circuits asbefore, but only one inter-- mediate frequency amplifier tube. l'n sucha case three tuned circuits may be used in each of the coupling systemscorresponding to systems 23 and' Sli of Fig. l; or if desired, fourtuned circuits might be used in one of the coupling systems and twotuned circuits in the other- Regardless of what particular arrangementof. tuned circuits is used it is highly desirable that at least three,and preferably four or more, tuned circuits b-e arranged in tandem i-nthe intermediate frequency amplifier, in any manner familiar to thoseskilled in the art. It maybe stated as a general proposition that some,normally from one-third to three-fourths of the total number of tunedcircuits, should be shifted in frequency by the device U".

From the foregoing it is seen that it is not essential that all of theadjustable circuits (or any two of them, for that matter) have equalrates of resonance shift. All that is required is thatv the resultantshift be suicient for the purpose herein set forth.

In a manner analogous to that described above, the lower sideband,instead of the upper sideband, could be extended, by moving themechanical control device U in the opposite direction.

The particular sideband which is chosen for expansion should ordinarilybe that which is most free from interfering signals. For example, if astrong interfering signal is present in the vicinity of the uppersideband, the lower sideband should be chosen for expansion.

Owing to the fact that the voltage output changes with the manipulationof device U' (e. g. curves B and D of Fig. 2) a system of automaticvolume control should be employed if it is desired to produce a moreuniform output. Any conventional automatic volume control system may beemployed, such as that shown in H. A. Wheeler Patent 1,879,863. Theautomatic volume control voltage should be derived from a point in thesystem subsequent to the intermediate-frequency amplifier system.

The sources of direct voltage for placing the tube electrodes in anoperative condition are not shown in detail, but are simply representedas B, C and Screen to indicate, respectively, the anode, the gridbiasing and screen voltages. No source of cathode-heating current isindicated, since any well-known method of cathode-heating may beemployed.

I claim:

l. In a superheterodyne receiver including means for modulating incomingsignals to produce intermediate carrier-frequency signals, a selectingsystem for selecting a band of frequencies including the intermediatecarrier frequency, and means for increasing the width of the selectedband to select a greater portion of 'either of the side bands ofmodulation comprising means for shifting said selected band in eitherdirection with respect to the carrier frequency by an amount which stillincludes said intermediate carrier frequency.

2. In a superheterodyne receiver including means for modulating incomingsignals to produce intermediate carrier-frequency signals, a selectingsystem for selecting a band of frequencies including the intermediatecarrier frequency, said selecting system being tuned to the intermediatecarrier frequency, and means for increasing the width of the selectedband to select a greater portion of either of the side bands ofmodulation comprising a variable reactive element associated with saidsystem for shifting the selected band i-n eitherv direction relative tosaid intermediate carrier frequency by an amount Willich still includessaid intermediate carrier frequency.

3. In asuperheterodyne receiver including means for modulating incomingsignals to produce intermediate carrier-frequency signals, a selecting-lsystem for selecting a band of frequencies including the intermediatecarrier frequency, saidA4 selecting system comprising a primary circuitand a secondary circuit coupled together, eachH of said circuits beingtuned to the intermediate carrier frequency and having associatedtherewith means f'or increasing the width of the selected band to selecta greater portion of either of the side bands of modulation comprising avariable reactive element to detune said circuits i-n the same directionand in either direction, from the intermediate frequency.

4. In a superhe-terodyne receiver including meansfor modulating incomingsignals to produce intermediate carrier-frequency signals, a selectingsystem for selecting a band of frequencies including the intermediatecarrier frequency, said selecting system comprising a primary ceiland asecondary coil coupled together, eachof said coilsV being tuned bycapacity to the intermediate carrier frequency, and means for increasingthe Width of the selected band to select a greater portion of either ofthe side bands of modulation comprising a variable condenser comprisinga part of said capacity and associated with each of said coils, and amechanical control device for simultaneously operating said variablecondensers to Vary the capacity thereof in the same direction, wherebysaid coils may each be detuned from said intermediate carrier frequency,in the same direction.

5. In a superheterodyne receiver including means for modulating incomingsignals to produce intermediate carrier-frequency signals, a selectingsystem for selecting a band of frequencies including the intermediatecarrier frequency, said selecting system comprising a primary coil and asecondary coil coupled together, each of said coils being tuned bycapacity to the intermediate carrier frequency, and means for increasingthe Width of the selected band to select a greater portion of either ofthe side bands of modulation comprising a Variable condenser included asa part of the capacity across each of said coils and a mechanicalcontrol device attached to said variable condensers, which when movedproduces equal capacity variations in the same direction, whereby saidsystem may be detuned from said intermediate carrier frequency in eitherdirection.

6. In a modulated carrier frequency signaling arrangement, a sidebandselecting system comprising a plurality of circuits each normally tunedto the carrier frequency, a portion only of said circuits includingadjustable reactance elements, and means for operating said reactanceelements to shift the resonant frequencies of the circuits which includesaid reactance elements, relative to the resonant frequencies of otherof said circuits, whereby the selected band may be expanded in eitherone direction or the other to select a Vgreater width of one of thesidebands of modulation.

7. In a modulated carrier frequency signaling arrangement, a carrierfrequency amplier having a rst pair of coupled tuned circuits at itsinput and a second pair of coupled tunedcircuits at its output, saidpairs of circuits being normally tuned to a desired carrier frequency,said first pair having an adjustable reactance element in each of itscircuits, mechanical control means for operating said reactance elementsin the same direction to shift the resonant frequency of said first pairrelative to the resonant frequency of said second pair, whereby theselected band may be expanded in either direction to select a greaterWidth of one of the sidebands of modulation.

8. In a superheterodyne receiver, an intermediate carrier frequencyamplifier comprising a plurality of selecting circuits each sharplytuned to said intermediate carrier frequency, and means for slightlyshifting in the same direction, the resonant frequency of some of saidcircuits in either direction relative to the resonant frequency of otherof said circuits, whereby the selected band may be expanded in onedirection or the other to select a greater Width of one of theintermediate frequency sdebands.

9. In a modulated carrier-frequency signaling arrangement, aband-selecting system comprising a plurality of circuits each normallytuned to the carrier frequency, and means for shifting the resonantfrequencies of a portion only of said circuits relative to the resonantfrequencies of other of said circuits, whereby the selected band may beexpanded in one direction or the other to select a greater Width of oneof the side bands of modulation.

10. In a modulated carrier-frequency signaling arrangement, aband-selecting system comprising a plurality of reactively coupledcircuits each normally tuned to the carrier frequency, and means forshifting the resonant frequencies of a portion only of said circuitsrelative to the resonant frequencies of other of said circuits, Wherebythe selected band may be expanded at one side of said carrier frequencyto select a greater portion of the frequencies of modulation at saidside of the carrier.

11. In a superheterodyne receiver including means for modulatingincoming signals to produce intermediate carrier-frequency signals, aselecting system for selecting a band of frequencies including theintermediate carrier frequency, and means for increasing the Width ofthe selected band to select a greater portion of the modulationfrequencies at one side of said intermediate carrier frequencycomprising means for shifting said selected band with respect to saidintermediate carrier frequency in the direction of said side by anamount which still includes said intermediate carrier frequency.

DANIEL E. HARNE'I'I.

